SOIL ORGANISMS 



429 



Three modes of nitrogen fixation are usually recognized : (1) 

 rain-water additions, (2) the action of soil organisms func- 

 tioning independently of living higher plants, and (3) the 

 influence of organisms functioning parasitically or symbi- 

 otically in the soil. 



236. Additions of nitrogen in rainwater. — Nitrogen oc- 

 curring in rainwater is generally in the nitrate and ammoni- 

 cal forms and, consequently, is readily available to plants. 

 The amounts thus brought down are quite variable, usually 

 fluctuating markedly with season and location. The follow- 

 ing table gives some of the more important findings regarding 

 the amount of nitrogen thus added to the soil in various parts 

 of the world. 



Table XCIV 



1 Russell, E. J., and Richards, E. H., The Amount and Composition 

 of Bain Falling at Bothamsted; Jour. Agr. Sci., Vol. IX, pp. 309-337, 

 1919. 



a Crowther, C, and Ruston, A. G., The Nature, Distribution and 

 Effects upon Vegetation of Atmospheric Impurities In and Near an 

 Industrial Town; Jour. Agr. Sci., Vol. IV, pp. 25-55, 1911. 



3 Von Feilitzen, H., and Lugner, I., On the Quantity of Ammonia 

 and Nitric Acid in Bainwater Collected Near Flahult, in Sweden; Jour. 

 Agr. Sci., Vol. Ill, pp. 311-313, 1910. 



4 Hudig, J., The Amounts of Nitrogen as Ammonia and Nitric Add 



